Fix sys/timerfd.h cbindgen.toml to generate proper C headers instead of C++.

The empty cbindgen.toml from P3-timerfd-relative.patch caused cbindgen to
generate C++ output (cstdarg, constexpr, etc.) in the installed
sys/timerfd.h. C compilers including this header would fail with
"cstdarg: No such file or directory". Add language="C", after_includes
for bits/timespec.h, and explicit export list for the timerfd constants.

diff --git a/src/header/sys_timerfd/cbindgen.toml b/src/header/sys_timerfd/cbindgen.toml
--- a/src/header/sys_timerfd/cbindgen.toml
+++ b/src/header/sys_timerfd/cbindgen.toml
@@ -1,12 +1,23 @@
 sys_includes = ["time.h"]
+after_includes = """
+#include <bits/timespec.h> // for itimerspec
+"""
 include_guard = "_SYS_TIMERFD_H"
 language = "C"
 style = "Tag"
 no_includes = true
 cpp_compat = true
 
 [enum]
 prefix_with_name = true
 
+[export]
+include = [
+    "TFD_CLOEXEC",
+    "TFD_NONBLOCK",
+    "TFD_TIMER_ABSTIME",
+    "TFD_TIMER_CANCEL_ON_SET",
+]
+
 [export.rename]
 "itimerspec" = "struct itimerspec"